1. Field of the Invention
The present invention relates to a driving circuit which drives light-emitting diodes, and in particular to a light-emitting diode driving apparatus which drives light-emitting diodes by using AC power supply.
2. Description of the Related Art
In recent years, significant attention is given to light-emitting diodes (hereinafter, occasionally referred to as “LEDs”) as lighting sources. The reason is that LEDs can be driven at low power consumption as compared with filament lamps or fluorescent lamps. LEDs are small, and have shock resistance. In addition, LEDs are less prone to blow out. Thus, LEDs have these advantages.
In the case of lighting sources, it is desirable that commercial AC power for home use is used as power supply for lighting sources. LEDs are devices driven by DC power. LEDs emit light only when applied with a current in the forward direction. Also, in the case of LEDs that are currently typically used for lighting use, the LEDs operate on DC power at a forward directional voltage Vf of about 3.5 V. LEDs do not emit light if a voltage applied to the LEDs does not reach Vf. Conversely, a voltage applied to the LEDs exceeds Vf, an excessive amount of current will flow through the LEDs. Accordingly, it can be said that DC power is suitable for driving LEDs.
To satisfy the contradictory conditions, various types of LED driving circuits have been proposed that use AC power. For example, a method has been proposed that switches LEDs so that a Vf total value is changed in accordance with a varying voltage value (see Japanese Patent Laid-Open Publication No. JP 2006-147,933 A). In this method, a number of LEDs connected to each other in series are divided into blocks 161, 162, 163, 164, 165 and 166 as shown in a circuit diagram of FIG. 16. The LED blocks 161 to 166 are selectively connected to the power supply in accordance with the voltage value of input voltage of rectified waveform by a switch control portion 167 consisting of a microcomputer so that a Vf total value is changed in a stepped manner. As a result, as shown by a voltage waveform in a timing chart of FIG. 17, since the LEDs can be driven by a plurality of rectangular waves corresponding to the rectified waveform, the LED usage ratio efficiency can be improved as compared with the ON-duty in the case of only single rectangular wave.
On the other hand, the applicant has been developed an AC multi-stage circuit which includes a plurality of serially-connected LED blocks operated by an AC current after full-wave rectification, each of the plurality of LED blocks having a plurality of serially-connected LEDs (Japanese Patent Laid-Open Publication No. JP 2011-40,701 A).
As shown in FIG. 18, this AC multi-stage circuit subjects a current from an AC power supply AP to full-wave rectification in a bridge circuit 2 so that the LED blocks of multi stages are supplied with the current after the full-wave rectification. As the LED blocks of multi stages, first, second and third LED blocks 11, 12 and 13 are serially connected to each other. A first LED current control transistor 21A is turned ON/OFF to connect/disconnect a first bypass BP1 which bypasses the second LED block 12 based on the current amount in the first LED block 11. A second LED current control transistor 22A is turned ON/OFF to connect/disconnect a second bypass BP2 which bypasses the third LED block 13 based on the current amount in the first and second LED blocks 11 and 12. This AC multi-stage circuit can keep power supply efficiency high, and additionally improve the LED usage ratio efficiency and the power factor.
FIG. 19 shows the current waveform of this AC multi-stage circuit. As shown in this figure, the current waveform has a stepped shape in synchronization with the power supply cycle. This stepped current waveform has a shape close to a waveform of a sine wave current. However, this current varies in a stepped manner, which in turn may cause harmonic interference. In the case where a filament lamp is used as load instead of LEDs, its current waveform will be a sine wave. For this reason, in this case, harmonic interference will not occur. Lighting apparatuses are classified into the class C in the IEC61000-3-2 standards. In the standards, the harmonic limit is specified. In particular, the limit for apparatuses of not smaller than 25 W is higher as compared with apparatuses of not higher than 25 W. From this viewpoint, it may be difficult for the AC multi-stage circuit shown in FIG. 18 to meet the limit.
FIG. 20 is a graph showing exemplary measurement data of harmonic current in a light-emitting diode driving method shown in Patent Laid-Open Publication No. JP 2006-147,933 A. As shown in this graph, the measured values in some harmonic orders, in particular 11th, 13th and 15th orders, exceed the limits, and do not meet the standards.
The present invention is devised to solve the above problems. It is a main object of the present invention to provide a light-emitting diode driving apparatus capable of suppressing harmonic components.